1. Field of the Invention
The present invention relates to a method for removing sulphur dioxide and possible hydrogen sulphide as well as nitric oxides from exhaust gases by means of sulphides in a liquid phase so that the sulphur is recovered in the form of molten elemental sulphur. The soluble sulphates created in the scrubbing are regenerated in a known fashion either by means of sulphides back into sulphides or by means of reduction, and are subsequently conducted back into the sulphur dioxide absorption stage.
2. Description of the Prior Art
The sulphur dioxide contained in exhaust gases can be removed in various different ways. So far the most popular method has been to neutralize SO.sub.2 with lime, limestone, magnesite, etc. The created sulphate/sulphite is a thixotropic waste which is only slightly soluble and difficult to treat. New methods for recovering SO.sub.2 are continuously being developed; one of the current trends has been to remove SO.sub.2 from exhaust gases by means of various sulphides.
The German Patent No. 913 889 discloses a method where the sulphur dioxide contained in exhaust gases is absorbed into a Na.sub.2 S-bearing solution. The molar ratio of sulphur dioxide is threefold compared to the mole quantity of sodium sulphide, and the purpose of the whole process is that all of the SO.sub.2 contents are associated into tetrathionate. The recovery of sulphur from tetrathionate is carried out by heating the solution in an open vessel. A quarter of the sulphur contained in the tetrathionate is released as sulphur dioxide and conducted into the hood above the heating vessel, and subsequently into the sulphur dioxide absorption stage. Two quarters of the sulphur contained in the tetrathionate are recovered in the form of elemental sulphur, and one quarter remains in sodium sulphate. The created sodium sulphate is regenerated by means of barium sulphide back into sodium sulphide and returned to the SO.sub.2 absorption stage. The created barium sulphate is reduced into barium sulphide by means of carbon. The sulphur generated during the process of breaking down tetrathionate is finely divided and is passed on for further cleaning.
Sherritt Gordon Mines has developed a method in which elemental sulphur and ferrosulphate are produced by dissolving the sulphur dioxide contained in exhaust gases into a suspension of pyrrhotite in water. This process is described in the German Patent Application No. 2 118 513 and in the Canadian Metallurgical Quarterly, Vol. 9, No. 4, p. 551-561. In the said method, both the sulphur dioxide and the pyrrhotite in aqueous suspension are conducted into an autoclave, where they are reacted into ferrosulphate and elemental sulphur according to their molar ratios. The temperature in the autoclave is between 65.degree.-121.degree. C., and the SO.sub.2 partial pressure is at least 1.05 kg/cm.sup.2. From the autoclave there is recovered solid elemental sulphur and nonreacted pyrrhotite as well as iron sulphate solution which can be utilized in the production of iron. In order to accelerate the reactions between pyrrhotite and sulphur dioxide and thus to increase the quantity of elemental sulphur, some carbon can be added into the autoclave.
As for the practical performance, the Sherritt Gordon process still has some difficulties: in the method SO.sub.2, and consequently the SO.sub.2 -laden gas likewise, are conducted directly into the autoclave. The process description does not mention anything about concentrating the SO.sub.2 -laden gases. If exhaust gases with a SO.sub.2 content between 0.05-10% are conveyed directly into an autoclave, this sets considerable requirements for the size of the autoclave. In the said publication it is, however, mentioned that SO.sub.2 is conducted into the autoclave in gaseous form. Thus it is not for example concentrated in a scrubbing solution in the scrubbers located before the autoclave.
The U.S. Pat. No. 4,078,048 describes the removal of sulphur dioxide from exhaust gases by means of an alkali metal compound, such as hydroxide. The exhaust gases are scrubbed in an aqueous solution of an alkali metal compound, which process generates for instance sodium sulphate/sulphite. The pH in the scrubbing stage is alkaline and fluctuates between 10-14, and the temperature varies between 50.degree.-60.degree. C. Thereafter, the sulphate/sulphite is regenerated by means of barium sulphide, so that in addition to barium sulphate precipitate, sodium sulphide is also created.
The created sodium sulphide is oxidized in the presence of air and a suitable catalyst in order to create elemental sulphur. The formed sulphur is flotated at the surface of the oxidation tank. The alkaline metal compound created in the oxidation is led back into the exhaust gas scrubbing. The BaSO.sub.4 created in the regeneration is reduced into barium sulphide by means of carbon.
There is also a known process where SO.sub.2 -bearing gases are scrubbed with ferrosulphide (FeS). This method is described in the German Patent Application No. 2836466 and in the publication Energy Technology Processes of the Energy Technology Conference, Decade of Progress, February 1983; 10(1983), p. 925-934. This method is called the Sulf-x process. In the said method the SO.sub.2 -bearing gases are scrubbed in a wet scrubber, where an aqueous suspension of finely divided ferrosulphide (FeS) is employed for absorbing SO.sub.2 from the gases. The solution obtained from the scrubber contains sodium sulphate, ferrosulphate and solids. The solids are separated from the solution. Part of the solution is conveyed into crystallization in order to remove the sulphates, and the rest is returned into scrubbing. The mother liquor received from the crystallization is also returned to scrubbing. The sulphate crystals and the solids separated from the scrubbing solution are combined and led into a reducing thermal treatment. The high-sulphur iron sulphide created in the scrubbing is broken down and the released sulphur is evaporated. The sulphur is recovered by means of condensing. The sodium sulphur contained in the solids is reduced into sodium sulphide. From the thermal treatment the solids are transferred back into the scrubbing solution, so that the sodium sulphide precipitates ferrosulphide out of the ferrosulphate and is itself reacted into sulphate.
It is preferable that the dissolving stage is carried out in such conditions (temperature 50.degree.-70.degree. C., pH 5.5 to 7.5) that the forming of iron oxide (Fe.sub.2 O.sub.3) and elemental sulphur is prevented, and only ferrosulphate and high-sulphur iron sulphide (FeS.sub.2) are created. The elemental sulphur is not formed until the reducing thermal treatment, where the temperature is about 760.degree. C. and the elemental sulphur is evaporated and is recovered by means of condensing.
In the method according to the U.S. Pat. No. 4,083,944, the SO.sub.2 -laden gas is absorbed into a buffer solution, which is for example a citrate solution. The absorption takes place in an absorption tower, and over 80% of the sulphur dioxide of the gases is absorbed into the citrate solution. The pH of the absorber solution is adjusted within the range of 4.0 to 4.5 in order to achieve an optimal result. After the SO.sub.2 absorption, sulphide ions are led into the solution at the regeneration stage, which sulphide ions are either in the form of gaseous H.sub.2 S or NaHS, so that the sulphide ion precipitates the elemental sulphur. The first stage for recovering the elemental sulphur is flotation. The solution obtained from this stage is conducted back into the SO.sub.2 absorption tower. The elemental sulphur obtained by means of flotation, which sulphur contains impurities and the rest of the absorber solution, is treated in an autoclave whre several layers are formed; the pure, molten sulphur forms the lowest layer, the impurities are located in the middle layer and the citrate solution is in the topmost layer. This citrate solution is also conducted into recirculation. If H.sub.2 S is utilized in the regeneration, it is made of the elemental sulphur created in the process; if NaHS is utilized, it is made by recycling part of the SO.sub.2 -bearing citrate solution into the reactor where concentrated CaS solution is added. CaS increases the pH value of the solution and simultaneously the NaHSO.sub.3 contained in the solution reacts with calcium sulphide so that NaHS and CaSO.sub.3 are created. The calcium sulphite is insoluble, and it is filtered from the NaHS-bearing citrate solution. The calcium sulphite is regenerated into sulphide for instance by reducing with carbon.